Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, when shutting down the engine (either during a key off event or as an engine start/stop event) hydrocarbons may result from unburned gases and rotation reversal of the engine may occur (resulting in engine shaking).
In one example, the issues described above may be addressed by a method for an engine comprising: in response to a request to shut down the engine: closing an intake throttle and opening a first valve disposed in a secondary flow passage coupled between an intake manifold, downstream of the intake throttle, and a first exhaust manifold coupled to a first set of exhaust valves to route unburned hydrocarbons to a catalyst disposed in an exhaust passage coupled to a second set of exhaust valves. For example, the request to shut down the engine may be a key off request. As a result of this operation, unburned hydrocarbons may be directed to the catalyst, thereby reducing hydrocarbons in the engine system and maintaining the catalyst at stoichiometry. Further, by closing the intake throttle, engine reversal is reduced. Additionally, by opening the second valve and the intake throttle after the engine has stopped rotating, less exhaust gas recirculation (EGR) may be pulled back into the intake.
As another example, the issues described above may be address by a method for an engine comprising: in response to a request to shut down the engine: deactivating all valves of a first set of exhaust valves configured to flow exhaust to an exhaust passage via a first exhaust manifold; and opening a first valve disposed in a flow passage coupled between an intake passage and a second exhaust manifold, where a second set of exhaust valves are coupled exclusively to the second exhaust manifold. For example, the request to shut down the engine may be a start/stop request generated responsive to the vehicle in which the engine is installed being stopped. By deactivating the first set of exhaust valves, gases may be recirculated via the second set of exhaust valves, thereby running down the pressure in the intake manifold. This may improve engine operation upon restart, at which time the first set of exhaust valves may be reactivated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.